This invention relates to three-phase motors, and more particulary to a three-phase delta-wound motor adapted for operation from a source of single-phase current.
In many rural areas, the demand for three-phase motors has increased dramatically because of the increasing mechanization and electrification of the farm. Many tasks which formerly were performed by hand are now performed by electrical motors, and many of these tasks require a large (i.e., 25+) amount of horsepower. As a practical matter, the only motors widely available in the larger horsepower sizes are three-phase delta-wound motors. Unfortunately, many rural areas do not have three-phase power service and the cost of extending this service to those areas is often prohibitive.
Some systems have been devised to address the problem of running three-phase motors where only single-phase current is available. For example, rotary converters are used on the farm to convert single-phase power to three-phase power. But if only a single three-phase motor is to be used, the cost of a rotary converter may not be economically justifiable. Static converters are often used to run individual three-phase motors from a single-phase source, but present static converters are also not without their limitations. An example of such a static converter system is described in U.S. Pat. No. 2,922,942. That particular system, however, is directed to three-phase wye-connected motors and is not useable with 240 V delta-wound or connected motors.
A further difficulty in using large horsepower motors in rural areas is that the maximum current available is relatively low. Although the amount of current available is usually sufficient to run even large (e.g. 50 hp) three-phase motors, it is often not enough to start such a motor. Motors such as that described in U.S. Pat. No. 2,922,942 typically require a starting current that is five to six times greater than the running current. Because of this, many rural power companies will not allow larger horsepower motors to be connected to their single-phase lines.